In a device, such as an air-conditioning apparatus equipped with a refrigeration cycle, there are cases in which a refrigerant stagnates in a compressor while the device is not in operation. For example, as is the case with an air-conditioning apparatus where a heat exchanger, which is a component of the air-conditioning apparatus, is disposed outdoors, viscosity of the lubricant oil in the compressor decreases along with drop of concentration due to dissolving of the refrigerant stagnated in the compressor to the lubricant oil in the compressor. When the compressor is started under such a condition, the lubricant oil having low viscosity is supplied to the rotating shaft and the compression unit of the compressor, creating risk of burnout due to poor lubrication. Furthermore, when a liquid level of the lubricant oil in the compressor increases due to the dissolving of the refrigerant, a starting load of the compressor increases, which is identified as an over current at the start-up of the air-conditioning apparatus, and a start failure of the air-conditioning apparatus is caused.
As a way to solve the above problem, there is a method in which stagnation of refrigerant in the compressor is suppressed by heating the compressor not in operation. As for the method of heating the compressor, there is a method of energizing an electric heater wound around the compressor, and a method of applying low voltage high frequency current to a coil of a motor installed in the compressor to heat the compressor by Joule heat generated in the coil without rotation of the motor.
That is, with the above method, the compressor is heated in order to prevent the refrigerant from stagnating in the compressor while not in operation, and, accordingly, power will be consumed even while the compressor is suspended. As a measure to this problem, a control method of suppressing the amount of power that is consumed to prevent the refrigerant from stagnating in the compressor is disclosed in which an outdoor air temperature detected by a temperature detecting means is used to determine if heating of the compressor is required, and when determined that heating is not required, the heating of the compressor is stopped (see Patent Literature 1, for example). Specifically, the compressor is heated when the outdoor temperature is equal to or below a predetermined temperature in which the refrigerant may stagnate in the compressor and when the temperature is equal to or below a predetermined temperature in which the compressor is deemed as not in operation.
Further, a control method of suppressing the amount of power that is consumed to prevent the refrigerant from stagnating in the compressor is disclosed in which a discharge temperature of the compressor detected by a temperature detecting means and a discharge pressure of the compressor detected by a pressure detecting means provided in the air-conditioning apparatus are used to estimate a state of the compressor, determining if heating of the compressor is required or not, and when determined that heating is not required, the heating of the compressor is stopped (see Patent Literature 2, for example). Specifically, the refrigerant saturation temperature is converted from the compressor discharge pressure. Then, when the compressor discharge temperature is equal to or below the refrigerant saturation temperature, it is determined that the refrigerant has been liquefied and has stagnated, and the compressor is heated.